FAILURE MODES OF PVD COATINGS IN MOLTEN Al-ALLOY CONTACT

Abstract

This paper deals with a study of the failure mode of thin PVD coatings in alternated contact withmolten aluminum alloy. CrN and ZrN monolayer coatings deposited through cathodic arcevaporation were used. The coatings morphology was assessed by SEM and their mechanicalproperties evaluated by nanohardness test performed at room temperature. An experimental testrig which cyclically immerses coated steel samples in molten Al-alloy and in a cooling bath wasapplied. The thermal gradient from the coating to the steel core was exalted by internal coolingchannels placed in the internal cavity of samples. Periodical SEM inspections were performed toassess the damaging levels introduced by the test and to study the related decrease of substrateprotection capability. Descriptions and interpretations of the damages evolutions were derived.The main conclusions achieved are that both coatings suffered by the formation of corrosionpits, which were due to a corrosion attack of the steel substrate localized at coating defects sites.In particular, at pores locations the corrosion was fast, whereas at droplets sites it required acertain incubation time. Once corrosion pits were formed they exhibited an initial tendency toexpand laterally, but they did rapidly stabilize in terms of lateral dimensions. Later on twodifferent failure modes acted in ZrN and in CrN. Extended delamination due to a markedmismatch of mechanical properties between the coating and the steel substrate developed inZrN. On the contrary, thermal cracking due to lower hardness levels developed in CrN, but withlimited delamination. Accordingly the steel substrate protection capability was evaluated to behigher in CrN than in ZrN.Keywords: PVD